1. Field of the Invention
The present invention relates to a constant-current source circuit employed in a CMOS semiconductor integrated circuit and, more particularly, to a self-activated constant-current source circuit.
2. Description of the Related Art
The conventional constant-current circuit is exemplified in the laid-open Japanese Patent Publication No. 293327/1987.
The constant-current source circuit disclosed above outputs constant current Ic at an output terminal OUT in accordance with power source voltage V.sub.DD, and incorporates a first and a second current mirror circuits connected to a power source voltage terminal and ground respectively, a resistor connected to the second current mirror circuit, and P channel type MOS FET (hereinafter called as PMOS transistor) connected to the first current mirror circuit.
The first current mirror circuit has current gain G1 greater than one.
The second current mirror circuit has current gain G2 which varies from 1 to 1/2 depending on a voltage drop across the resistor.
Since the resistor passes the current of the second current mirror circuit to ground, the voltage drop across the resistor varies depending on current variation of the second current mirror circuit.
The resistor constitutes a closed-loop circuit in combination with the first and the second current mirror circuits.
The PMOS transistor outputs constant current Ic at an output terminal OUT in accordance with the current passing through the first current mirror circuit.
This PMOS transistor constitutes a current mirror circuit in combination with the PMOS transistor in the first current mirror circuit.
The above mentioned constant-current source circuit is connected to an activation device which generates an external activation signal.
This activation device includes a detector for detecting supply of power source voltage V.sub.DD and a trigger generator for generating an external trigger activation signal in response to the detected result of the detector.
To activate the constant current source circuit, the power source voltage V.sub.DD is applied to a prescribed terminal and the external activation signal is applied from the outside activation device to the first and the second current mirror circuit, whereby the constant-current source circuit operates in a stabilized condition at the operating point where the product of the current gain of the first current mirror circuit by the current gain of the second current mirror circuit determined by the resistor equals to one.
On this instance, the current passing in the closed-loop circuit, which is constituted by the first and the second current mirror circuits and the resistor, can be outputted at the output terminal OUT as the constant current Ic through the current mirror circuit which is constituted by the PMOS transistor and the PMOS transistor of the first current mirror circuit.
In the constant-current source circuit described above, when two NMOS transistors of the second current mirror circuit are operated in a weak inversion region by adequetly designing those gate widths and gate lengths, the product of the constant current Ic and the resistance value of the resistor is proportional to the absolute temperature.
Accordingly, when the constant-current Ic passes through a resistor, the voltage drop across the resistor is in proportion to absolute temperature.
As the result, the constant-current source circuit can output constant current Ic and can also be easily incorporated into CMOS integrated circuits as a standard voltage source having low power consumption.
The constant-current source circuit described above has, however, following problems to be solved.
Since the external signal is provided when the constant-current source circuit is to be initiated, the activation device must be incorporated to generate the external signal.
The activation device comprises a detector for detecting power source voltage V.sub.DD and a trigger generator for outputting the external signal, which is complicated in structure.
The activation device has another problem that if power source voltage V.sub.DD varies slowly when the power is supplied, the power supply cannot be detected.
On this occasion, since the closed-loop circuit including the first and the second current mirror circuits and the resistor does not function stably so as to generate constant-current Ic, no current passes in the closed-loop circuit, that is, all of PMOS and NMOS transistors in the first and the second current mirror circuits turn to be "off" state.
Another activation method for the constant-current source circuit has been proposed which utilizes parasitic capacitance of PMOS and NMOS transistors constituting the first and the second current mirror circuits.
In this method, however, since the activating operation depends on when power source voltage V.sub.DD is applied, the activating operation may fail depending on how power source voltage V.sub.DD is supplied.
There is also a possibility that the activating operation becomes unstable due to manufacturing variation of parasitic capacitors.